A few closely related camptothecin derivatives, and their analogs have been reported to be useful in the treatment of cancer. Some of such compounds described in the prior art are outlined below:    (i) EP publication no. 0074256A1, U.S. Pat. Nos. 4,473,692 & 4,545,880 disclose compounds of formula (III)
wherein R1 stands for a hydrogen atom, an alkyl group, a hydroxyl group, an alkoxy group or an acyloxy group; R2 for a hydrogen atom, an alkyl group, an aralkyl group, a hydroxymethyl group, a carboxymethyl group or an acyloxymethyl group, and R3 for the grouping —XR′ (where R′ is a hydrogen atom, an alkyl group or an acyl group and X is an oxygen atom or a sulfur atom), a nitro group, an amino group, an alkylamino group, an acylamino group or a halogen atom, with the proviso that when both of R1 and R2 are hydrogen atoms, R3 should not be hydroxyl group, methoxy group or acetoxy group.
An example of these compounds is shown in formula (IIIa)
    (ii) GB patent no. 2056973 discloses compounds of formula (IV)
wherein X is H, CH2OH, COOH, an alkyl group, an aralkyl group or the grouping CH2OR1 or CH2OR2 wherein R1 is an alkyl group or an acyl group and R2 is a lower alkyl group, Y is H, OH or the grouping OR3 wherein R3 is a lower alkyl group or an acyl group, and Z is H or an acyl group, with certain combinations excluded, and water soluble alkali metal salts thereof, are prepared by introduction of a substituent at X or Y optionally followed by modification of those and/or other substituents.
An example of these compounds is shown in formula (IVa)
    (iii) The European Publication no. EP 321122 discloses topotecan of formula (III), which is already marketed
